The present invention refers to methods in the fabrication of metal pipes and concerns in particular a method for the fabrication of a metal pipe having a wall with at least one smooth portion and at least one grooved portion.
Methods are already known (see, for example, the documents U.S. Pat. No. 629,245, U.S. Pat. No. 3,487,673 and U.S. Pat. No. 3,744,290) for the fabrication of a metal pipe with a grooved portion, in which the grooved portion is obtained from, an initially smooth portion by means of progressive deformation of the wall of the smooth portion in at least two consecutive phases, where each phase is performed with the help of a tool assembly including a grooved female die, designed to receive the portion of pipe being deformed, and a grooved male punch, axially movable within the aforesaid portion of pipe and defining a plurality of longitudinal ribs distributed around the pipe in positions that are circumferentially spaced apart from each other.
The known methods, however, have the drawback of not enabling the production of a pipe structure having an elevated resistance to deformation, both flexural and torsional, and at the same time a relatively reduced weight.
The purpose of the present invention is to realize a method of the above specified type that is able to overcome the drawbacks of the known solutions and in particular guarantees the production of a pipe having optimal characteristics.
A further purpose is to realize a method that can be implemented with simple equipment and at relatively reduced cost, and with quick and easy steps, enabling an increased productivity.
In order to accomplish the aforesaid purpose, the method according to the invention has all the characteristics that were indicated in the beginning of the present specification and furthermore is characterized by the fact that the majority of the aforesaid ribs formed by means of the aforesaid progressive deformation of the initially smooth portion of pipe each have a width in the circumferential direction that is significantly smaller than the distance in the circumferential direction from one rib to another and by the fact that subsequently to the said progressive deformation of the said portion of pipe, the latter is subjected to a further operation of deformation in order to press into contact with each other the flanks of each rib, at least for the greater portion of said ribs.
Thanks to this characteristic, the method according to the invention guarantees the production of a structure having the desired resistance, without detriment to the characteristics of lightness and in particular without requiring the use of a pipe of elevated thickness, which brings obvious advantages also from the standpoint of manufacturing cost.
According to another preferred characteristic, both at the end of said progressive deformation of said portion of pipe (1a), and alter said further deformation step to press into contact with each other the flanks of each rib, the wall of the pipe maintains a cylindrical shape between one rib and another.
Further preferred characteristics of the present invention are indicated in the appended claims, which are an integral part of the teaching provided herein.